This invention relates to electrochemical cells which can be used as power sources for storage and release of electrical energy. In particular, this invention relates to electrochemical cells such as, but not limited to, batteries, capacitors and hybrid electrochemical cells termed batcaps. The latter exhibit characteristics of both a battery and a capacitor. More particularly, this invention relates to electrochemical cells which accomplish the conversion of chemical energy to electrical energy at ambient temperature by using a non-liquid electrolyte in which protons are mobile, which cells require no thermal activation for their operation.
Electrochemical cells including batteries, capacitors and batcaps are useful for storage and/or release of electrical energy and use similar electrolytes and electrodes. They differ, however, in the mechanisms used for energy storage and their discharge characteristics.
In rechargeable batteries, stored chemical energy is converted into electrical energy almost entirely via reversible charge transfer reactions of active materials occurring mainly in the electrode bulk. The double layer that exists at the surface of the electrodes contributes only a minor amount to the total energy.
In conventional capacitors the electrodes are made of materials which essentially do not participate in charge transfer reactions and so almost all the energy is stored in the double layer at the surface of the electrodes. However, in electrochemical capacitors, electrodes are made of materials which can participate in reversible charge transfer reactions, and so a large portion of energy is also contributed by these reversible charge transfer reactions, occurring mostly at the surface of the electrodes.
Regarding discharge characteristics, electrochemical capacitors, as compared to rechargeable batteries, are typically characterized by lower energy density, higher power density, shorter capacity retention time, and greater cycle life.
A batcap has discharge properties which can be characterized as intermediate between those of batteries and those of electrochemical capacitors. Reducing the thickness of a rechargeable battery results in very thin electrodes. Such an ultra-thin battery can be considered a batcap since the ratio of electrode bulk to electrode area is diminished and its power density increases. When high currents are used in the operation of such a thin electrochemical cell the charge transfer reactions occur mainly at the surface of the electrodes and the cell can be considered to be a batcap.
Batteries have been developed which contain a solid rather than liquid electrolyte, since these exhibit practical advantages such as a high form factor, thin, flat, flexible shapes and avoidance of fluid leakage or drying out. However, some of these batteries employ electrodes composed of metals, such as palladium (see for example U.S. Pat. No. 4,894,301), which are expensive, or materials which may be dangerous to health and difficult to manufacture. Other batteries release hydrogen ions from a metal alloy or metal hydride anode material in a liquid electrolyte battery such as a nickel-metal hydride cell. Other batteries require thermal activation in order to release hydrogen ions from the anode via deintercalation of protons from the anode (see for example U.S. Pat. No. 4,847,174).
In the past, aromatic nitro compounds were considered for active battery cathode materials in non-rechargeable batteries and only for liquid aqueous electrolytes (see for instance U.S. Pat. No. 2,306,927, Dec. 29, 1942, U.S. Pat. No. 3,025,336, Mar. 13, 1962, R. Glicksman and C. K. Morehouse, J. Electrochem. Soc., 105 (1958) 299 and R. Udhayan and D. P. Bhatt, J. Electrochem. Soc., 140 (1993) L58). Since these compounds are reduced irreversibly under these conditions, they are not suitable for rechargeable batteries. In addition, these compounds suffer from one or more of the following deficiencies: low cell voltages, toxicity, significant solubility in the electrolyte, instability with regards to the electrolyte, poor shelf-life, high self-discharge, and low power density.
Further prior art considered the halogenated organic compounds for active battery cathode materials because of their generally higher voltage but only in non-rechargeable batteries and only for liquid aqueous electrolytes (U.S. Pat. No. 2,874,079, Feb. 17, 1959 and R. Udhayan and D. P. Bhatt, J. Electrochem. Soc., 140 (1993) L58). Besides the disadvantages mentioned above for the aromatic nitro compounds, the halogenated compounds also suffer from being corrosive, producing chlorine odors and are difficult to handle. In still more recent prior art, some quinone compounds have been used as anodes in liquid aqueous electrolyte batteries (see for instance H. Alt, et. al., Electrochim. Acta, 17 (1972) 873 and F. Beck, et. al., The Electrochemical Society Abstracts, No. 152, October 1994 Meeting). However inherent deficiencies limit their applicability in practical batteries. These electrode materials are not stable with respect to the liquid electrolyte and so they degrade. In addition, these electrode materials are soluble in liquid electrolytes and so the integrity of the electrodes is significantly diminished and there is high self-discharge and poor shelf life. Furthermore, they are not useful in practical batteries because their voltages are generally too low.